How to take control of your 401K: self directed 401k

[Clinton Leighton]

I’m interested in taking control of my 401K and using the funds to finance my real estate investments.  I see this as a good idea for two reasons.  1.) it will help me on my path to financial freedom through real estate. 2.). It will help insulate my retirement account from a volatile stock market.  
Does any one have thoughts on this matter?  Has anyone already done this?  Can anyone point me in the right direction?